Brainstem External Features
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Direct answer
Midbrain, pons and medulla display, on their exterior, the landmarks from which every cranial nerve below the third emerges. The midbrain shows two cerebral peduncles flanking the interpeduncular fossa in front, the four colliculi of the tectum behind, and gives origin to the third nerve between the peduncles; the trochlear is unique in emerging on the dorsal aspect, below the inferior colliculus. The bulging belly of the pons carries the trigeminal nerve on its lateral aspect and the sixth, seventh and eighth nerves along its lower border, while the medulla exhibits pyramids, olives, the pyramidal decussation, the gracile and cuneate tubercles, and the rootlets of nerves nine to twelve in strict rows.
What you must remember
- Midbrain anteriorly: the basis pedunculi of each cerebral peduncle flanking the interpeduncular fossa, with its posterior perforated substance and the oculomotor nerve emerging between them.
- Midbrain posteriorly: superior and inferior colliculi (corpora quadrigemina); the trochlear nerve emerges lateral to the midline below the inferior colliculus — the only cranial nerve to leave the brainstem dorsally, after decussating in the superior medullary velum.
- Pons anteriorly: transverse pontine fibres sweeping back on each side as the middle cerebellar peduncle; the basilar artery rides the basilar sulcus; the trigeminal nerve emerges by a large sensory and small motor root on the anterolateral surface.
- Pontomedullary junction: from medial to lateral — abducent nerve (between pons and pyramid), facial nerve with the nervus intermedius, and vestibulocochlear nerve at the cerebellopontine angle.
- Medulla anteriorly: pyramid beside the anterior median fissure, with the pyramidal decussation visible where most fibres cross; the olive lies lateral to the pyramid, and hypoglossal rootlets leave the sulcus between them.
- Medulla posterolaterally: glossopharyngeal, vagus and cranial accessory rootlets emerge from the retro-olivary (postolivary) sulcus between the olive and the inferior cerebellar peduncle.
- Medulla posteriorly: gracile tubercle medially and cuneate tubercle laterally, opening above into the floor of the fourth ventricle; striae medullares cross the floor at the pontomedullary boundary.
- Cerebellar peduncles: inferior (between medulla and cerebellum), middle (from the pons, the largest), and superior (roofing the fourth ventricle).
Localising from the surface
A woman arrives with vertigo, tinnitus and deafness on the right; months later her face weakens on the same side, and later still the cornea is anaesthetic. The surface anatomy gives the answer before any scan. The vestibulocochlear nerve and the facial nerve with its nervus intermedius emerge together at the cerebellopontine angle, where a schwannoma of the vestibular division grows and presses neighbouring structures in order of proximity: eighth nerve first (hearing loss, tinnitus, vertigo), then the facial nerve (ipsilateral palsy, dry eye, loss of taste on the front of the tongue), then the trigeminal nerve higher in the angle (anaesthetic cornea, lost corneal reflex), and finally the inferior cerebellar peduncle (ipsilateral limb ataxia). A large tumour compresses the fourth ventricle, causing hydrocephalus. The reverse logic works on the anterior surface: an aneurysm at the vertebrobasilar junction presses the abducent nerve where it emerges between pons and pyramid (diplopia with internal squint), while a clival meningioma devours the sixth nerve first because it has the longest subarachnoid run to the cavernous sinus.
Where students slip
Two rows get scrambled in the viva: hypoglossal rootlets leave between pyramid and olive (pre-olivary sulcus), while IX, X and the cranial root of XI leave behind the olive (retro-olivary sulcus) — the olive is the divider. The second slip is the trochlear: it is the thinnest cranial nerve, the only one to emerge from the dorsal brainstem, the only one to decussate completely, and it supplies the superior oblique, the muscle that depresses the adducted eye. Candidates also forget that the facial nerve has two roots — motor and nervus intermedius — visible only at the cerebellopontine angle.
Frequently asked questions
Which cranial nerves emerge at the cerebellopontine angle?
The facial nerve (with the nervus intermedius) and the vestibulocochlear nerve. A vestibular schwannoma here first causes unilateral hearing loss and tinnitus, then facial weakness and trigeminal sensory loss.
Where does the trochlear nerve emerge, and why is it exceptional?
It is the only cranial nerve to emerge from the dorsal aspect of the brainstem, below the inferior colliculus of the midbrain, after its fibres decussate in the superior medullary velum. It is also the thinnest cranial nerve and supplies only the superior oblique.
Which structures lie on either side of the olive?
Hypoglossal rootlets emerge anterior to the olive (pre-olivary sulcus); the glossopharyngeal, vagus and cranial accessory rootlets emerge posterior to it (retro-olivary sulcus), in front of the inferior cerebellar peduncle.
What are the gracile and cuneate tubercles?
Swelling on the posterior medulla produced by the gracile and cuneate nuclei, where the dorsal columns of the same side synapse. They are the medulla's surface marker of the conscious proprioception pathway.
Which peduncle connects pons to cerebellum, and what does it carry?
The middle cerebellar peduncle (brachium pontis) carries the pontocerebellar fibres — the second limb of the corticopontocerebellar pathway from the opposite half of the pons to the cerebellar hemisphere.